Electronically enhanced transmission systems have been well developed in the prior art as may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,595,986; 4,648,290; 4,722,248; and 5,050,427, the specifications of which are hereby incorporated by reference in their entirety. Transmission systems such as these have been utilized to provide a variety of gear ratios to enhance the flexibility and torque multiplication of an engine to service a plethora of applications. The most common applications include MVMA Class 7 and Class 8 tractor semi-trailer vehicles although other applications, such as automobile or stationary power plant powertrains, may also be serviced.
An electronic control module which includes a microprocessor is often used to control the powertrain, which includes an engine as well as a multiple gear ratio transmission. The continuous evolution of microprocessor technology has enabled increased accuracy and expanded the scope of control over engine and transmission operations. The electronic control module collects data from various sensors and issues commands appropriate for the current operating conditions to control the engine and transmission. Engine control may include modulating fuel, operating engine accessories, or managing application of an engine retarder, driveline retarder, or both. Transmission control may include selection of an appropriate gear ratio, including disengagement of the current gear ratio and engagement of a new target gear ratio, or operation of an input shaft brake.
Efficient ratio changing improves fuel economy and enhances drivability of a vehicle. Under certain demanding situations, such as when negotiating a steep grade with a heavily loaded vehicle, swift ratio changes are required to prevent the vehicle from losing momentum and missing entirely the window of opportunity to complete the shift. Under normal driving conditions, an operator may have to shift gears more than fifteen times before reaching highway speeds. In these applications, inefficiency in ratio changing may accumulate to a significant amount of wasted time. Thus, it is desirable to reduce the time necessary to complete a ratio change or shift.
A typical ratio change involves a number of steps. First, the operator must interrupt the transfer of torque from the engine through the transmission to the driveline. This may be accomplished by disengaging a master clutch which provides a frictional coupling between the engine and the transmission. The master clutch may be controlled by a modulating actuator in response to an appropriate command signal initiated by the operator, the electronic control module, or both in cooperation. Likewise, a simple (discrete or "dumb") actuator having only engaged and disengaged states may be used. Alternatively, a "throttle dip" may be performed where the throttle is abruptly decreased. Once the torque transfer has been interrupted, the current gear is disengaged and the transmission is in a neutral state.
The next step in a typical ratio change involves selecting the target gear ratio. This may be the next available gear ratio in a sequence, or a number of available ratios may be skipped, depending on the current operating conditions. Before engaging the target gear, the transmission input shaft should rotate at a substantially synchronous speed for the current output shaft speed and target gear ratio. When the master clutch is engaged, the input shaft speed may be manipulated by controlling engine speed since the engine and transmission are coupled. Engine speed may be increased (for a downshift) or decreased (for an upshift) to realize synchronous speed. On transmissions equipped with an input shaft brake, the input shaft speed may be reduced by disengaging the master clutch and applying the input shaft brake (also known as an inertia brake or clutch brake). However, input shaft brakes with sufficient capacity to decrease ratio changing time add cost and complexity to the transmission system and require accurate sequencing of events for satisfactory operation, so many transmissions only utilize simple versions of these devices.
For transmissions without input shaft brakes, synchronous speed will not be attained on an upshift until the engine speed naturally decays to synchronous. As engines and transmissions become more and more efficient, the reduction of internal frictional losses results in substantially lower natural decay rates. This results in a correspondingly longer time to complete a ratio change. Thus, it is desirable to increase engine and/or transmission input shaft deceleration during an upshift to achieve synchronous speed shortly after disengagement of the current gear.
When the master clutch is disengaged for a ratio change, engine speed and input shaft speed will likely decay at different rates based on their respective inertias. Thus, it is desirable to cooperatively control the decay rates of the engine speed and the input shaft speed to reduce the ratio changing time based on current operating conditions. This may be accomplished by retarding engine rotation, transmission input shaft rotation, or both. Likewise, any device or component coupled to the input shaft or engine during the ratio change may be retarded to improve the ratio changing time. Likewise a power synchronizer may be utilized to increase input shaft speed in conjunction with increased fueling to increase engine speed to decrease ratio changing times for a downshift.
One device often utilized to provide a variable retarding force to an engine, is an engine brake. The most common engine brakes may be either engine compression brakes or exhaust brakes. These devices are well known in the prior art and are commonly provided on heavy-duty vehicles. Examples of vehicular automated mechanical transmission systems utilizing engine brakes may be seen by reference to U.S. Pat. Nos. 4,933,850 and 5,042,327 the specifications of which are hereby incorporated by reference in their entirety.
Engine compression brakes are usually manually operated and provide a variable retarding force resisting engine rotation by altering valve timing of one, two, or three banks of cylinders. This creates compressive force within the cylinders which resists rotation of the crankshaft. Exhaust brakes operate in a similar fashion by restricting exhaust flow from the engine. Exhaust brakes do not offer the responsiveness or flexibility of engine compression brakes although they are less expensive to employ.
Traditionally, engine brakes are utilized to assist the vehicle service brakes by supplying a resisting torque on the driveline when descending long grades. Manual operation of the engine brake in these situations continues to be a desirable option. More recently, engine brakes have been manually operated to decrease the time required for ratio changes. For this application, manual operation of the engine brake often results in large torque disturbances to the vehicle driveline due to inappropriate timing in applying and releasing the engine brake. This reduces drivability of the vehicle and may also adversely affect the durability of powertrain components. Furthermore, proper operation is largely dependent upon the skill and experience of the vehicle operator.
A driveline retarder may also be used alone or in combination with any of the retarding devices described above. Driveline retarders are typically pneumatically, hydraulically, or electromechanically operated to impart a retarding force on the driveline, typically the drive shaft or prop shaft of a rear-drive vehicle.